Reduced numbers of ETPs correlate to thymic hypoplasia in Smad4^{co/co;Foxn1Cre} mice. (A) Absolute number of thymic ETPs (Lin⁻CD25⁻CD44⁺Sca-1⁺c-kit^hi cells) in mice of different ages. (B) Analysis of developmental progression from the DN1 to the DN2 stage in Lin⁻c-kit^hi thymic progenitors. (C) Quantification of proliferation in DN thymocytes 4 hours after BrdU administration and determination of intrathymic transition kinetics 4 days after the BrdU pulse in SP thymocytes. DN1 and DN2 populations were identified as c-kit^hi CD25^neg and c-kit^hi CD25^pos, respectively. (D) Linear correlation of ETP numbers with total thymic cellularity in mice ranging in age from 1.5 to 15 weeks. (E) Relative numbers of thymic ETPs in mice of different ages. (F) Total thymus cellularity and cell numbers of ETP, DN2, and DN3 populations in 6-week-old mice. DN2 cells were identified as c-kit^hi CD44⁺CD25⁺. A minimum of 3 mice per group were analyzed at each time point. Values are mean plus SD. □ represents Smad4^co/co; ■, Smad4^{co/co;Foxn1Cre}. *P < .05, **P < .01, ***P < .005, ****P < .001.